An image intensifier tube is a vacuum tube device that increases the intensity of available light in an optical system to facilitate visual imaging of low-light processes, such as fluorescence of materials in x-rays or gamma rays (x-ray image intensifier), or for conversion of non-visible light sources, such as near-infrared or short wave infrared to visible.
Image intensifiers based on micro-channel plate (MCP) and proximity focus concept can provide high gain due to MCP magnification, low distortion, and uniform resolution across an entire field of view. However, MCP-based image intensifiers tend to have relatively bad resolution for many critical applications. In addition, MCP may block as much as 40% of the photoelectrons right after the photocathode. Thus, detective quantum efficiency for MCP-based image intensifiers is usually low.
To achieve higher detective quantum efficiency, intensifier tubes based on electrostatic focusing lens or combined magnetic-electrostatic focusing optics may be utilized. Such image intensifier tubes usually have much better detector quantum efficiency (DQE) and resolution than MCP-based image intensifiers. However, electron and photon scattering in the amorphous phosphor scintillating layer can still degrade the final resolution. In addition, fiber plate or relay optical lens is required to transfer the light emitted on the phosphor screen to the final imaging device, such as CCD or CMOS. Resolution and gain can be further degraded at this coupling stage. To collect as much light as possible, high numerical aperture (NA) relay lens may be required. High NA and large field of view (FOV) optics require a relay lens with a large diameter and long profile. The cost of such relay optics may become significant. The shallow depth of focus in such a collection scheme is another concern. All these shortcomings increase the challenges of optical alignment and field service.
To overcome these issues in intensifier based detectors, pixelated image sensors such as CCD or CMOS sensors are placed on the phosphor screen location to directly collect photoelectrons emitted from the photocathode. These kinds of detectors are typically referred to as electron bombarded CCD (EBCCD) detectors or electron bombarded CMOS (EBCMOS) detectors. EBCCD or EBCMOS devices eliminate the electron-to-photon conversion step in phosphor screen and the expensive coupling device between phosphor and CCD or CMOS sensor.
Most current EBCCD/EBCMOS detectors are designed based on proximity focus method to simplify the design, reduce the power requirements and make the detector compact. Proximity focus EBCCD/EBCMOS are disclosed, for example, in U.S. Pat. No. 5,321,334 issued on Jun. 14, 1994 to Katsuyuki Kinoshita and Yoshinori Inagaki, and in U.S. Pat. No. 6,285,018 issued on Sep. 4, 2001 to Verle W. Aebi et al.
A conceptual drawing of proximity-focus EBCCD is shown in FIG. 1. A photocathode layer 101 is coated on a glass substrate 100. A CCD/CMOS chip 104 is placed on a package substrate 105 facing photocathode 101. The whole package is sealed by potting material 103 to form a vacuum tight tube. Photocathode in traditional EBCCD/EBCMOS device is usually in the form of transmission mode. It means incoming photons will pass through the glass window and illuminate the photocathode layer on the side with interface layer to the glass substrate. Upon the incoming photon illumination, photoelectrons 102 are emitted from the vacuum side surface of the photocathode layer, then they will be accelerated by the bias voltage 106 applied between the photocathode layer and the sensor surface.
When photoelectrons are emitted from the photocathode, their initial velocity usually has component normal to the photocathode plane and component parallel to the photocathode plane. The velocity component parallel to the photocathode plane will create lateral spread of the electron cloud originated from the same spot on the photocathode plane. The extent of the lateral spread is proportional to the initial lateral velocity and the traveling time between the photocathode and the CCD/CMOS sensor. To reduce the lateral spread, it is important to reduce the initial lateral speed and reduce the traveling time. Initial lateral speed is determined by the incoming photon energy, photocathode work function and the band gap structure. Traveling time between the photocathode and the sensor is determined by the gap and the accelerating voltage between them. A narrower gap and higher accelerating bias voltage will result in shorter travel time, thus better resolution. However, narrower gap and higher bias voltage means higher electric field strength between the photocathode and the sensor. If the electric field strength approaches 2˜4 kV/mm, the risk of arcing increases significantly depending on the vacuum pressure, surface smoothness and materials. To get a sub-pixel resolution, the gap needs to be so small that non-flatness of CCD/CMOS chip, especially on the back-thinned sensor becomes significant. Such a non-uniform gap may results in variation of resolution, localized distortion and increased risk of arcing.
If the gap can't be reduced, bias voltage has to be increased to improve resolution. However, higher energy electrons inside sensor will increase the X-ray yield and damage the CCD/CMOS sensor by increasing dark current and hot pixels and reducing gain due to increased defect density. To improve the lifetime of the EBCCD/EBCMOS sensor, it's better to keep the landing energy of the photoelectrons on the CCD/CMOS chip lower than 1 or 2 keV. Therefore, there is a conflicting requirement between improving lifetime and improving resolution on proximity focus EBCCD/EBCMOS. To achieve high gain at low landing electron energy, boron coating instead of oxide coating is applied on back-thinned EBCCD/EBCMOS. Boron coated back-illuminated sensor has been disclosed in U.S. Published Patent Application No. 2013/0264481 published on Oct. 10, 2013 to Jehn-Huar Chen et al.
To improve resolution, electrostatically focused hybrid EBCCD design has been disclosed in U.S. Pat. No. 5,321,334 issued on Jun. 14, 1994 to Katsuyuki Kinoshita et al and a research paper published in Nuclear Instruments and Methods A, issue 2-3, page 255, August 1998 by S. Buontempo et al. However, electrostatic focused vacuum tube usually has poor focus uniformity or non-flat object/image plane and high image distortion. Such shortcomings limit its application in high resolution Time Delay Integration (TDI) imaging sensors. For example, distortion may be rendered as blur in TDI mode imaging sensors.
Another attempt to improve EBCCD/EBCMOS resolution is disclosed in U.S. Pat. Pub. No. 2013/0148112A1 published on Jun. 13, 2013 to Yung-Ho Alex Chuang et al. The disclosed method involves insert a focusing plate with a micro-lens array between the photocathode and the sensor. However, many photoelectrons emitted from the photocathode will likely be blocked by the closed area on the focusing plate. Thus this approach may reduce the detective quantum efficiency (DQE) of the whole EBCCD/EBCMOS.
The overall DQE of an EBCCD/EBCMOS device is mostly determined by the quantum efficiency (QE) of the photocathode. In a transmission-mode photocathode, photons are mostly absorbed on the front side of the photocathode. Then the energetic electrons inside the photocathode layer need to diffuse to the vacuum side of the photocathode before they can escape the energy barrier created by the work function. Momentum of the energetic electrons may be lost during the diffusion process between the two surfaces. In a reflective mode photocathode, photons are absorbed on the vacuum side of the photocathode. Energetic electrons can immediately escape the photocathode close to the same location. Therefore, a reflective mode photocathode usually has significantly higher quantum efficiency.
It is well-known that reflective mode photocathode can achieve more than 50% to 100% higher quantum efficiency (QE) compared with corresponding transmission mode photocathode. For example, a research paper published in Proceedings of SPIE vol. 8359 in 2012 by Yoshihiro Ishigami, et al compared the QE of GaN photocathode in reflective mode and transmission mode. The QE of GaN photocathode for 266 nm photons can be as high as 37% in reflective mode. Yet the QE will be reduced to 17% in transmission mode. It's almost impossible to implement the reflective mode photocathode in traditional proximity-focus EBCCD/EBCMOS without significantly sacrificing resolution by increasing the tube length. A reflective mode oblique magnetic field focused EBCCD/CMOS device had been reported by C. B. Opal and G. R. Carruthers in the Proceedings of SPIE vol. 1158, page 96-103 in 1989 to improve the resolution and quantum efficiency. Such a device has a magnetic field tilted with respect to the accelerating electric field axis. The oblique magnetic field can deflect the photoelectrons off the normal axis and focus them on to the sensor that is not located on the normal axis. The overall device is bulky. Focus aberrations and geometrical distortion in oblique focus design could be too high for many high resolution TDI imaging applications, such as semiconductor defect inspection equipment.
What is needed is an EBCCD/CMOS device that can achieve high spatial resolution, low landing energy, and high gain. Furthermore there is a need for an EBCCD/CMOS device that can achieve these requirements even if the sensor has many tens of microns or about one hundred microns of non-flatness.